The Conservatives are suffering from their reckless immigration pledge

Theresa May had better hope that she was wrong. “People will judge us on whether we have done what we said we would do. What people will say at the next election is, ‘Have they made a difference? Did they actually get on with doing what they promised?’ ” Mrs May was talking about that immigration promise: to bring net migration down to under 100,000 “over the term of a parliament”. Four years on, new ONS figures show how far away that ambition is: net migration rose by 58,000 in the year to September 2013, to 212,000.

You can picture the posters now: "David Cameron said he'd get immigration under 100,000. It's now over 200,000. Vote Ukip." As these are the last migration figures before the European elections, they are very awkward for the Conservatives. Already, only 17 per cent trust the Tories most on immigration. Mr Cameron has kept his promise to protect pensioners' perks – despite pressure to the contrary – explaining that when "you make these very clear public promises, you should keep them". Plenty of those who voted for him in 2010 will be asking why this rationale doesn't apply to immigration. Blaming it all on Lib Dems, who prevented David Cameron's "tens of thousands" pledge becoming enshrined in the Coalition Agreement, will hardly suffice.

Mr Cameron's folly was making a promise that he couldn't possibly keep. The latest figures include net EU migration of 131,000 – numbers that the Government has no control whatsoever over. All the tub-thumping on immigration has opened up a credibility gap, between what the Government espouses on the subject and on what the electorate thinks that it's actually capable of delivering. This contributes to the collapse in public trust that afflicts all three main papers, and an underlying factor in Ukip's rise.

It didn't have to be this way. The number of Italians with new National Insurance number registrations rose 18,000 on a year earlier; the number of Spaniards with new National Insurance number registrations rose by 14,000. These people are migrating because, although Britain has an appalling productivity record (output per hour worked in the UK is 21 per cent lower than the other six members of the G7), it has a jobs record the envy of the European Union: unemployment is five per cent lower than the EU average.

A rise in immigration is one inevitable result of this success, but the Tories' intemperate rhetoric means that no one will recognise it as one. Having immigration and growth is better for the Conservatives than having neither. But those cack-handed promises on immigration have spectacularly backfired – and could eat into any growth dividend that the party gets with the electorate. Given that the polls have been static for seven months, it's one that the Tories badly need.